Express Middleware Review

## What is Middleware? - **Middleware** is a function that executes during the lifecycle of a request to the server. - Middleware has access to the **request object** (`req`), the **response object** (`res`), and the **`next` function**. - Common uses: logging requests, handling errors, parsing incoming data, and serving static files. - Middleware is executed in the order it is added to the app. ### How Middleware Works 1. A request is sent to the server. 2. Middleware functions process the request. 3. If middleware calls `next()`, control passes to the next middleware or route handler. 4. If middleware does not call `next()`, the request-response cycle stops. ```javascript app.use((req, res, next) => { console.log('Request URL:', req.url); console.log('Request Method:', req.method); next(); }); ``` - `app.use()` adds middleware globally to the application. ## Application-Level Middleware - **Application-level middleware** is bound to an instance of the Express app. - Registered using `app.use()` or route-specific methods like `app.get()`, `app.post()`, etc. - Executes for every incoming request unless restricted to a specific route. - Use cases: logging, authentication, and error handling across multiple routes. ```javascript app.use((req, res, next) => { console.log('Request received at:', new Date()); next(); }); ``` ## Router-Level Middleware - **Router-level middleware** is bound to an instance of `express.Router()` instead of the main app. - Applied using `router.use()` and only affects routes handled by that router. - Helps modularize the app into isolated route modules. - Mount a router with `app.use('/path', router)` to scope the middleware to specific routes. ```javascript const router = express.Router(); router.use((req, res, next) => { console.log('Request made to /menu route'); next(); }); router.get('/drinks', (req, res) => { res.send('Welcome to the drinks menu!'); }); app.use('/menu', router); ``` ## Error-Handling Middleware - **Error-handling middleware** is a special middleware used to catch errors during the request-response cycle. - It has **four parameters**: `err`, `req`, `res`, and `next`. The four-parameter signature is what tells Express this is an error handler. - Invoked when an error is passed to `next(err)`. - Should be placed **at the end** of the middleware stack, after all other middleware and routes. ```javascript app.use((err, req, res, next) => { console.error(err.stack); res.status(500).send('Something went wrong!'); }); ``` ## Built-in and Third-Party Middleware ### Built-in Middleware - **`express.json()`**: parses incoming requests with JSON payloads. ```javascript app.use(express.json()); ``` - **`express.static()`**: serves static files such as images, CSS, and JavaScript. ```javascript app.use(express.static('public')); ``` - **`express.urlencoded()`**: parses incoming requests with URL-encoded payloads (the format sent by HTML forms); makes parsed data available on `req.body`. ```javascript app.use(express.urlencoded({ extended: true })); ``` ### Third-Party Middleware - **`cors`**: enables Cross-Origin Resource Sharing. ```javascript app.use(cors()); ``` - **`morgan`**: logs HTTP requests. ```javascript app.use(morgan('tiny')); ``` - Both built-in and third-party middleware functions are added in the order they are called with `app.use()`.